1. Field of the Invention
The invention is directed to a hydraulic system particularly adaptable for use with an excavator or other large earthmoving machines, having a hydraulic pressure relief system which can be independently actuated by the operator of the machine to reduce hydraulic line pressure and in turn reducing temperature.
2. Description of the Prior Art
Excavators and other earthmoving machines use hydraulic systems to power and locate numerous operative members, such as boom members, buckets, lifting members, dozer blades, etc. In addition, hydraulic systems can be used to drive the machines through hydrostatic drives. Forcing hydraulic fluid through valve bodies at high pressures drops increases the temperature of the fluid. The oil is heated while passing through the valve body in both the pumping path and the exhaust path. Typically, the exhaust oil is cooled after it passes the directional control valve in the hydraulic fluid sump on the machine. Normally, the sump will be provided with an oil cooler for assisting in this task.
Hydraulic pressure relief systems are well known and are used to protect the hydraulic system from overload by directing hydraulic fluid to the sump if excessive overpressure is developed. FIG. 2 illustrates such a system, wherein two pressure relief valves 10 and 12 are mounted in parallel between exhaust hydraulic line 14 containing a pressurized hydraulic fluid and hydraulic fluid sump 16. In operation, hydraulic fluid is pumped from sump 16 by pump 18 and directed by pumping line 20 through directional valve 22 to hydraulic motor 24. The exhausted hydraulic fluid is returned through line 14 to sump 16. Three-position directional valve 22 can be used to stop the flow through the hydraulic lines or redirect the flow from the pump in the lines downstream of the valve thereby effectively reversing the hydraulic motor. The positioning of valve 22 is controlled by a pilot hydraulic control system that forms a source of secondary hydraulic pressure. Hydraulic fluid from pump 72 directs fluid from sump 76 through line 25 to pilot control actuator 60. A second hydraulic line 27 is coupled to sump 76. By manipulating actuator 60, the positioning of valve 22 is controlled.
The hydraulic pressure relief system is also provided with a snubber valve 26 having a conventional port and a restricted orifice. If excessive hydraulic pressure is developed, the pressure of hydraulic fluid in sensing line 28 of first pressure relief valve 10 would overcome spring 30 opening this valve and releasing pressure through line 32 to sump 16. If the pressure continues to build, the second pressure relief valve would open, and as this is the bigger of the two valves, line 14 would effectively become depressurized. In opening the second pressure relief valve, snubber valve 26 is shifted to the right overcoming spring 33, so that the restricted orifice reduces the pressure in line 35 downstream of the snubber valve, this pressure drop caused by flow over the first pressure relief valve and reduces the biasing pressure on the second pressure relief valve to facilitate its opening. The excessive pressure is sensed through sensing line 34 of pressure relief valve 12 overcoming the biasing of spring 36 and the reduced hydraulic biasing of the fluid in hydraulic line 38 opening valve 12. Anti-cavitation check valve 40 is provided to prevent cavitation in line 14 by allowing fluid to flow from sump 16 to this line.